It is known to use a waveguide to detect a disturbance. However, such known techniques are not always suitable in situations where it is important to detect the position of a disturbance with a high spatial resolution, in particular when the area within which the disturbance should be detectable is a large area.
According to one aspect of the present invention, there is provided a position sensor including: an optical waveguide; a transmission stage for launching a sensing signal into the waveguide; a receiving stage arranged to receive a returned sensing signal, which returned sensing signal is a time distributed signal derived from backscattered components of the sensing signal, the waveguide being arranged along a path having a plurality of overlap regions such that a disturbance in an overlap region causes a first disturbance feature and a second disturbance feature in the returned sensing signal; and, monitoring means for monitoring the returned signal, such that a respective time of return can be associated with the first disturbance feature and with the second disturbance feature.
From the respective return times associated with the first and second disturbance features, the overlap region from which the disturbance originate can be identified more accurately than by using the return time of the first or the second disturbance only.
The disturbance features may be distinguished visually and their return times noted, for example from a trace signal plotted as a function of time on a display. From the return times, a user may then perform a computation to determine in which overlap region the disturbance has occurred. Alternatively, an electronic or optoelectronic circuit may be used to record the respective return times and to determine, from the return times, the origin of the disturbance. One example of a disturbance feature may be a change in the amplitude of the returned signal, for example a step change with respect to time.
The waveguide will preferably be sensitive to disturbances within a sensing region around the waveguide, the extent or width of the sensing region being dependent on the sensitivity of the position sensor. The overlap regions will then preferably be determined by the area of overlap between, on the one hand, the sensing region of one portion of the waveguide and, on the other hand, the sensing region of another portion of the waveguide. The different waveguide portions will preferably intersect or otherwise overlap in an overlap region. However, the different waveguide portions need not themselves overlap, but may instead come sufficiently close to one another such that their respective sensing regions overlap in an overlap region.
Preferably, the path includes: a plurality of first sensing portions; a plurality of second sensing portions; and a plurality of respective self-crossing points, where each first sensing portion overlaps with each second sensing portion. The first sensing portions will preferably generally extend side by side in a first direction. Likewise, preferably, the second sensing portions will generally extending side by side in a second direction.
Preferably the first and second sensing portions will be arranged in generally straight lines. However, the sensing portions may follow a meandering or curved path, the first and second directions being determined by the respective start and end locations of each sensing portion. In one embodiment, the respective directions of the first and second sensing portions will be substantially orthogonal to one another. However, the angle between the direction of first and second sensing portions may be less than 90 degrees, for example about 30 degrees.
The self-crossing points may be disposed as a two dimensional array or along a one dimensional path, which path may or may not be a straight path. The array may be a regular array where the self-crossing points occur at regular intervals along the fibre path or at regular geographical intervals. If the array is a two dimensional regular array, the intervals in a first direction may be different to those in a second direction. However, for simplicity and to make it easier to determine the spatial location of a disturbance, the intervals will preferably be equal in both directions.
The sensing signals will normally have a resolution time associated therewith, which, if the signals are pulses, will be the duration of a pulse. A path resolution length can be associated with the resolution time, this being the spatial extend of the pulses as the pulses travel along the waveguide. Preferably, at least some neighbouring self-crossing points will be connected together by a plurality of paths, for example both a short path and a long path. The long path between at least some of the neighbouring self-crossing points will preferably be longer or substantially equal to the path resolution length, to more easily allow neighbouring self crossing points to be resolved. Preferably, the geographical separation of at least some of the neighbouring points will be less than the path resolution length, allowing the disturbances to be spatially resolved on a length scale that is less than the path resolution length. Yet more preferably, to reduce the length of waveguide required, the short path between at least some of the neighbouring self-crossing points will itself be less than the path resolution length.
The long portion between neighbouring self-crossing points situated near the centre of the array is likely to be longer than that between neighbouring self-crossing points near the array edge. For some neighbouring self-crossing points towards an edge of the array, the long path could even be less than the path resolution length, making such neighbouring points difficult to resolve. To alleviate this situation, a respective extension stage may be provided between adjacent first sensing portions and/or between adjacent second sensing portions. Such an extension stage will extend the length of the long path length. In one embodiment, the extension stages are each formed by a respective coiled portion of waveguide. Preferably, the extension stages will each be longer than the path resolution length, to make it easier for self-crossing points near or on the edge of the array to be resolved. However, in some situation, in may not matter that the self-crossing points on the edge of the array are not resolvable, in which case the extension stage may have a path length that is less than the path resolution length.
Each extension stage will preferably be formed from an extension portion of waveguide, the extension portion being integrally formed with the adjacent waveguide sensing portions. In one embodiment, each of at least the first sensing portions will be integrally formed, preferably with the extension portions if such extension portions are included. Preferably, the first and second sensing portions will be formed from a continuous waveguide.
Each extension stage may be protected from disturbances, so that detected disturbances can more clearly be associated with a self-crossing point. For example, if the waveguide is buried underground so as to extend across a sensing zone in which disturbances are to be located, each extension stage may be located outside the sensing zone where a disturbance is unlikely to occur, and/or may be shielded from disturbances by a protective region, such as a region formed from a concrete material or other hard material. Alternatively, the extension stages may be used to detect the presence of an entity entering the sensing zone. Since the extension stages will each be formed from a coil of fibre, the extension stages are likely to be more particularly sensitive.
Preferably, the sensing signal will be formed as a pair of signal copies from an optical source. A dynamic or other time-varying physical disturbance is likely to produced a strain or an elastic wave in the optical medium of the waveguide, thereby changing the relative phase of the signal copies travelling through the disturbance. Preferably, the signal copies of a given pair will be launched onto the waveguide with a temporal offset to one another, such that there is a leading copy and a trailing copy. As a result, the signal copies of a pair are likely to be differently affected by a disturbance. The temporal offset may be undone upon the return of the signal copies, preferably by delaying the leading copy relative to the trailing copy. Both copies of a given pair can then be combined substantially in step with one another, so as to form the returned sensing signal. Because the signal copies are combined, any modification of at least one of these copies is likely to produce a change in the returned signal, thereby facilitating the detection of a disturbance.
A dynamic disturbance may be stationary, that is, located at a stationary point. Alternatively, the dynamic disturbance may be a moving disturbance, such as that produced by a person, vehicle or other entity moving along a surface.
The source will preferably be configured to transmit optical pulses, each pulse giving rise to a pair of pulse copies. The pulses will preferably be transmitted in a repeat fashion, at time intervals. The time intervals will preferably be chosen in dependence on the total path length of the waveguide, such that the round trip time to the far end of the waveguide is less than the time intervals between signals. Each pulse may thus give rise to an associated time-distributed return signal, the duration of the return signal being commensurate with (twice) the transit time of a pulse along the waveguide path. To reduce the risk that the return signal from different pulses will overlap, the pulses will preferably be transmitted at time intervals, the duration of the intervals being longer than or substantially equal to the transit time of the pulses along the path, yet more preferably longer than or substantially equal to twice the transit time.
Preferably, the sensing signal will be returned predominantly by a process of Rayleigh backscattering. Being a distributed backscattering process, the Rayleigh backscattering will cause the sensing signal to be returned progressively as the signal propagates along the waveguide. However, other distributed backscattering processes due to the properties of the light guiding medium along (which properties are present in a substantially continuous fashion along the length of the waveguide) may also contribute to the return signal, since such processes will also normally give rise to a returned signal that is distributed over time.
The output signals from the source will preferably have an irregular component, in which case the irregular component may be common to each of the signal copies of a pair. Other characteristics of the signal need not be the same in each signal copy: for example, the signal copies may have different amplitudes. The irregular component will preferably be random, or pseudo random (by pseudo random, it is meant that although in theory a component is possible to predict, the time or processing power required to do this will make it in practice impossible to predict). If the output signal has a waveform, the irregular component may be provided by the phase of the waveform if the waveform has randomly occurring phase changes. The waveform may conveniently be provided by an optical source having a short coherence time, preferably less than 10 pico seconds or even less than 1 pico second. The returned signal will preferably be an interference signal resulting from the interference or mixing of two waveforms.
A copying stage may be provided to copy the output from the source, and a combination stage for combining returned signal copies. In a preferred embodiment, the copying stage and the combination stage will be provided by a common optical junction. In particular, an interferometer stage, such as an un-balanced Mach Zehnder interferometer may be provided. In this preferred embodiment, the output from the optical source is fed to the interferometer, where the signal is copied, one copy being channeled to one path of the interferometer, the transit time associated with each path being different, such that a relative or differential delay results between the time at which the signal copies are transmitted from the interferometer stage. The same interferometer stage can then be employed to re-align the returned signal copies in a particularly convenient manner, since the relative delay imposed in the outbound direction will be the same as the relative delay imposed in the return direction, this being in each case determined by the difference in the transit times of the two paths.
The differential delay will preferably be chosen in dependence at least in part on the average coherence time of the source. The differential delay will preferably be much longer than the coherence time. Preferably, the ratio of the differential delay to the coherence time will be greater or equal to 102 or 103. The differential delay will preferably be at least 50 micro seconds, as it has been discovered that a long differently delay is beneficial in the detection the low frequencies typically associated with acoustic vibrations due to mechanical disturbances brought about my the movement of persons of vehicles along the ground surface. However, detection of low frequency disturbances can be improved yet further with a differential offset time of at least 75 micro seconds, or even 100 micro seconds (100 micro seconds corresponding to a distance of around 20 km for the differential path length of the interferometer if the interferometer legs are formed by silica glass single mode optical fibre).
The steps of copying output signals and transmitting the signals will preferably be carried out at a first location, a disturbance remaining detectable at distance of at least 1 km or even at least 10 km from the first location. The path length of the waveguide of the position sensor may be 100 km or more, allowing areas on the scale of at least several square km to be sensed, with the sensing points being placed reasonably close to one another.
According to another aspect of the present invention, there is provided method of sensing position, including the steps of: (i) transmitting a sensing signal along a waveguide; (ii) monitoring a returned sensing signal, the returned sensing signal being derived from components of the sensing signal that are returned by a process of distributed backscattering as the sensing signal propagates along the waveguide, wherein the waveguide is arranged along a path having a plurality of overlap regions such that a disturbance in an overlap region causes a first disturbance feature and a second disturbance feature in the returned sensing signal, which first and second disturbance features correspond to respective spaced apart positions along the fibre path; and, (iii) using a temporal characteristic in the first and second disturbance features to determine which of the overlap regions the disturbance originates from.
The returned signals will preferably be monitored as a function of the elapsed time from a reference time, which reference time will be related to the time at which the pulse is generated. A temporal characteristic in the returned signal may be a return time associated with the disturbance feature, in particular the arrival time of returned signal copies (once combined) responsible for a disturbance feature. Thus the return time may be the round-trip time (subject to a possible offset) for light propagating to and from the position of the physical disturbance.